Heretofore, during the cure of urethane prepolymers, phase separation patterns would occur. These phase separation patterns appear as relatively opaque white spots on a translucent white background of the cured urethane test plaque. The presence of these opaque white spots is visual evidence that the cured urethane is not homogeneous or of uniform quality throughout the plaque. When centrifugally casting large urethane parts, these opaque white spots are distorted and appear as flow lines in the cured urethane; that is, opaque white, broad lines on a background of translucent white. These opaque broad lines have no gross voids and are therefore not attributable to occluded gases.